Reduction of the moisture content of tobacco leaves during the curing process results in shrinkage of the leaf structure and a decrease in filling power. Shredding or cutting of the cured tobacco leaves to prepare filler may result in lamination and compression of the tobacco, thereby leading to an even greater filling power decrease.
Many procedures have been devised for increasing the filling power of cured tobacco. In particular, a large number of processes have been developed whereby the tobacco is impregnated with a solid, liquid or gaseous impregnant which, when removed during a subsequent expansion process step, generates elevated pressure in the tobacco cells, thereby expanding the cell walls. Such processes provide tobacco products of substantially increased filling power with an associated reduction in the total amount of tobacco used in, e.g., the preparation of a cigarette rod.
Most of the known processes, however, do not guarantee that the expanded bulk volume of the filler tobacco is retained over long periods of time such that, after casing, the firmness of the tobacco rod as packed would not be lost. For example, it is known that puffing of tobacco constituents through the application of high pressure steam followed by a sudden release of pressure does not necessarily lead to the retention of their expanded form. Loss of the expanded state and firmness over time is believed due to plasmolysis and cell contraction occurring during drying stages, which tend to cause the expanded tobacco to return to its original form.